To be more specific, what was being offered wasn't Starlight Energy itself, but a Natural Treasure from heaven and earth that seemed capable of converting Starlight Energy. Its name made its function pretty obvious. It was called Star Devourer, and it literally converted real stars into starlight energy directly!
Let's not forget that the twins' group wasn't the only one that used Starlight Energy for cultivation. In fact, Starlight Energy wasn't only used for body-related techniques either. Although it was a lot rarer, there were races that cultivated normally with Starlight Energy instead of Divine Energy. It's just that when fighting, Divine Origin Energy was still the main choice, even for races that cultivate with Starlight Energy, due to the abundance of the former.
From the description of the treasure, it could convert the energy of entire Class F Stars, which were the smallest stars, like a Red Dwarf, in a matter of one year, more or less.
